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The Vapor-Oil Gravity Drainage (VOGD) is a low temperature, solvent-enhanced gas-oil gravity drainage (GOGD) process targeting naturally fractured viscous reservoirs. The experimental set up and corresponding acquired data was previously introduced by the authors (Anand et al., 2017) in which the effects of temperature, solvent injection rate, and solvent type (n-Butane and dichloromethane (DCM)) were investigated. Results from Anand et al. work indicated encouraging high oil rates and ultimate recoveries; results also demonstrated that the oil rates and recovery were impacted by diffusion and dispersion (in the form of intrinsic gas rate), asphaltene precipitation, and capillary pressure. The intent of this work is to further study the mechanisms behind VOGD; in particular those related to operating pressure and solvent vapor-oil capillary pressure. The results from this work show that the ultimate recovery and oil rate are positively correlated to the operating pressure; experiments conducted at 50% and 75% saturation pressure (Psat) yielded lower ultimate oil recoveries, ranging from 33% to 68% of original oil in place (OOIP), when compared to the experiments conducted at 90% Psat (70% of OOIP). Moreover, n-butane performed better than DCM and lesser asphaltene precipitation was seen at lower Psat. The main drivers for these observations were found to be lower solvent solubility and larger capillary pressure values at lower values of Psat. |
